With the advent of a deep reactive ion etching (DRIE) process for forming slots and trenches in a semiconductor substrate, greater precision and control over the etching of silicon substrates in higher speed processes has been obtained. DRIE is a dry etching process carried out under high vacuum by means of a chemically reactive plasma, wherein the constituents of the plasma are selected in congruence with the substrate to be acted upon. Before the adoption of DRIE techniques to form trenches or slots in semiconductor substrates, most trenches or slots in substrates greater than about 200 microns thick were formed by mechanical blasting techniques or chemical wet etching techniques. However, such mechanical techniques or chemical wet etching techniques are not suitable for newer products that demand higher tolerances and smaller fluid flow features. DRIE enables deep anisotropic etching of trenches and slots with greater tolerances and without regard to crystal orientation.
DRIE techniques have progressed incrementally towards a goal of etching high aspect ratio features in semiconductor substrates wherein the aspect ratio is on the order of 1:100 width to depth. The process scheme for achieving high aspect ratio slots or trenches in semiconductor substrates includes a series of sequential steps of alternating etching and passivation. Such aniosotropic etching techniques are described in U.S. Pat. Nos. 5,611,888 and 5,626,716 to Bosch et al. the disclosures of which are incorporated herein by reference.
Most dry etching systems are designed to etch substantially vertical wall slots and trenches in the substrate, i.e., walls that are substantially perpendicular to a surface of the substrate. However, for micro-fluid ejection heads, it has been found that substantially vertical walls may entrap more air in fluids passing through relatively narrow slots. Such air entrapment can lead to fluid starvation for ejection devices on a device surface of the substrate. Accordingly, slots having reentrant wall angles are preferred. However, etching slots having reentrant wall angles increases the occurrence of etching defects in the substrate.
In order to prevent etching in areas adjacent to the areas to be etched, an etch mask is applied to a device surface of the substrate. Occasionally defects occur at or near the surface of the substrate adjacent to the etching location. Such defects may enable etching radicals to diffuse into a gap between the substrate surface and the etch mask applied to the substrate surface causing damage to the substrate surface on the device side of the substrate, hereinafter referred to as “device side damage.” Device side damage may also be caused by complete removal of a passivating layer along side walls in the slot location during the etching process. FIG. 1 is a photomicrograph of a portion of a device side of a semiconductor substrate 10 containing device side damage 12 adjacent to a fluid flow slot 14 therein. Despite advances made in the formation of slots and trenches in semiconductor substrates using a dry etch process alone, there continues to be a need for an improved process which provides desired slot wall angles while reducing the occurrence of device side damage to the semiconductor substrate.
With regard to the foregoing, there is provided a method of micro-machining a semiconductor substrate to form one or more through slots therein. The semiconductor substrate has a device side and a fluid side opposite die device side. The method includes diffusing a p-type doping material into the device side of the semiconductor substrate in one or more through slot locations to be etched through a thickness of the substrate. The semiconductor substrate is then etched with a dry etch process from the device side of the substrate to the fluid side of the substrate so that one or more through slots having a reentrant profile are formed in the substrate.
In another embodiment there is provided a process for etching a semiconductor substrate from a device side thereof to a fluid side thereof using a dry etch process to form at least one reentrant fluid flow slot therein. The process includes doping a portion of the device side of the substrate with a p-type doping material whereby device side silicon damage adjacent at least one fluid flow slot is effectively reduced.
An advantage of the process disclosed herein is that the process is capable of providing precisely formed slots having a reentrant profile while reducing the occurrence of device side damage to the substrate.